Optical reflectivity and carrier localization in incommensurate misfit layer compounds (MS)xTaS2 (M=rare-earth metal, Pb, Sn).

Abstract

The optical reflectance spectra of incommensurate misfit layer compounds (MS${)}_{\mathit{x}}$${\mathrm{TaS}}_{2}$ (M=rare-earth metal, Pb, Sn) were measured in the range of 3.1 to 0.093 eV and were analyzed by the Drude-Lorentz model. The spectra show clear Drude edges whose energies were significantly lowered from that of the host material 2H-${\mathrm{TaS}}_{2}$. The charge transfer from MS to ${\mathrm{TaS}}_{2}$ was estimated on the basis of the rigid-band model. We found that the effective mass of carriers in the ${\mathrm{TaS}}_{2}$ conduction band depended on the degree of charge transfer from MS to ${\mathrm{TaS}}_{2}$, which was explained by the modification of the electronic structure from a two-carrier to a one-carrier system with respect to the degree of charge transfer. The degree of charge transfer depending on rare-earth elements is related to the modification of the band structure of MS due to the lattice incommensurability between the MS and ${\mathrm{TaS}}_{2}$ sublattices, which is the most peculiar nature of (MS${)}_{\mathit{x}}$${\mathrm{TaS}}_{2}$.